Question

I have a special use case which I do not yet know how to cover. I want to dissect a string based on field_name/field_length. For that I define a regex like this:

'(?P<%s>.{%d})' % (field_name, field_length)

And this is repeated for all fields.

I have also a regex to remove spaces to the right of each field:

self.re_remove_spaces = re.compile(' *$')

This way I can get each field like this:

def dissect(self, str):
    data = { }
    m = self.compiled.search(str)
    for field_name in self.fields:
        value = m.group_name(field_name)
        value = re.sub(self.re_remove_spaces, '', value)
        data[field_name] = value
    return data

I have to perform this processing for millions of strings, so it must be efficient.

What annoys me is that I would prefer to perform the dissection + space removal in a single step, using compiled.sub instead of compiled.search, but I do not know how to do this.

Specifically, my question is:

How do I perform regex substitution combining it with named groups in Python regexes?

Answer 1

I take it each field sits next to each other in the string, like in a table, e.g.:

name     description        license
python   language           opensource
windows  operating system   proprietry

So assuming you know in advance the length of each field, you can do it much more simply, without using a regex at all. (btw, str is not a good name for a variable since it clashes with the builtin str type)

def dissect(text):
    data = {}
    for name, length in fields:
        data[name] = text[:length].rstrip()
        text = text[length:]
    return data

Then, if fields = [('lang', 9), ('desc', 19), ('license', 12)]:

>>> self.dissect('python   language           opensource')
{'lang': 'python', 'license': 'opensource', 'desc': 'language'}

Is this what you’re trying do though?